1. Field of the Invention
The present invention relates to piezoelectric vibrators such as a resonator used as a timing element, discriminator, filter or the like, and fabricating methods thereof. The present invention relates particularly to a laminated piezoelectric vibrator that is compact and has high and stable oscillating frequency, and fabricating methods thereof.
2. Description of the Related Art
A reference clock for the IC is required in order to utilize an integrated circuit (IC) which is a major element of various electronic devices. According to the development of IC technology, the various electronic devices are particularly controlled by single large scale integration (LSI) circuits, such as one-chip microprocessors. Most microprocessors have used ceramic resonator as a timing element. Since a ceramic resonator is stable, non-tunable and compact and is fabricated with low costs, its application is gradually becoming wide.
Lately, as performance and speed of the electronic devices is improved, it is required that oscillating frequency of resonators increases. Thus, it is required that the resonator to be developed hereafter has higher oscillating frequency to generate stable oscillation. Compact resonators are also required as the electronic devices become compact.
However, as shown in FIG. 1, a conventional ceramic resonator comprises a single plate type ceramic piezoelectric body 101 polished to have the thickness corresponding to desired frequency after sintering it, electrodes 102 formed on the upper and lower surfaces of the piezoelectric body 101 by thin film forming process, insulating cover layers 103 in which vibration grooves 104 are formed, and external electrodes 105 formed outside the piezoelectric body and cover layers. In such a case, the electrodes 102 are formed with various shapes so that energy trap occurs. The fabricating method of the conventional ceramic resonator is shown in FIG. 2. First, the piezoelectric body 101 is fabricated by pressing and molding ceramic powder for a piezoelectric body into quadrangle using general powder molding methods. The piezoelectric green body is sintered, cut with a dicing saw, and polished until it has thickness corresponding to desired frequency. On the piezoelectric body 101 processed to have the proper thickness, the electrodes 102 are formed by sputtering. Then, polling process of the piezoelectric body is performed. The polling process, which gives the ceramic piezoelectric body piezoelectricity, causes electric dipole to be oriented to one direction by applying electric field to the electrodes of the ceramic piezoelectric body. As shown in FIG. 2(b), the cover layers 103, in which the vibration grooves 104 are formed, are bonded to the upper and lower portions of the piezoelectric body, on which the electrodes are formed, by using epoxy 110. The cover layers are fabricated by sintering a green body that is formed of piezoelectric or dielectric ceramic powder by the general powder molding method. In such a case, the cover layer is formed with the mold, which is designed so that the vibration groove is formed on the cover layer as shown in the drawing. After the piezoelectric body and the upper and lower cover layers are bonded, a unit chip element 106 is obtained by cutting the bonded body as shown in FIG. 2(c). The external electrodes 105, each of which is connected to each of the electrodes 102 formed in the unit chip element 106, are formed outside the chip element as shown in FIG. 2(d). FIG. 1 is a cross-sectional view showing the chip element taken along the cross-sectional plane I-I. Then, the ceramic resonator is completed by epoxy molding or SMD (Surface Mount Device) packaging the chip element. Since an oscillating frequency in a resonator having MHz band is inversely proportional to its thickness in order to increase the oscillating frequency, the thickness of the piezoelectric body must be thin. Thus, in order for a single plate type piezoelectric resonator to have high oscillating frequency, the thickness of the piezoelectric body must continuously decrease by polishing. However, since there are many problems in practical manufacturing process, such as difficulty in polishing piezoelectric body plate uniformly over its entire surface, and breakage in handling, the conventional method is faced with the limitation to fabricating a resonator with high frequency. Accordingly, the production rate decreases and the production cost increases.
For the reasons, a higher harmonic vibration resonator, which utilizes high order vibration of a single plate type piezoelectric body, has been developed.
A general filter having MHz band uses energy trap due to a thickness vibration or thickness shearing vibration. If electrodes are formed on the entire surface of a piezoelectric substrate, it is difficult to obtain excellent resonance characteristics from combination with higher harmonic vibration of profile vibration. However, if the electrodes are formed partially on the surface of the piezoelectric substrate, there are boundaries between the portion in which the electrodes exist and the portion in which the electrodes do not exist. Since standing wave of the vibration occurs in the electrode portion near the boundaries, energy trap, which captures vibration energy, also occurs. The energy trap provides independent resonance characteristics although plural electrodes are formed on a substrate at intervals of predetermined or above space. In the case that the energy trap is used, the thickness shear vibration is utilized within the frequency range of 2 to 8 MHz, while thickness vibration is utilized within the frequency range of 8 to 16 MHz. However, since practical applications such as a mobile communication terminal, CD-ROM, HDD, and the like require high frequency of over 20 MHz, the resonator using third and fifth order vibrations of the thickness vibration has been utilized. That is, the higher harmonic vibration of wavelength 2L/n (n=integer) including a fundamental wave of wavelength 2L occurs in a piezoelectric substrate of thickness L. Since the vibrations wherein the n is an even number are cancelled by each other, the only vibrations wherein the n is an odd number appear. That is, the vibrations of third order, fifth order, seventh order, or the like including the fundamental wave occur. Resonance frequencies of the higher harmonic vibrations occur at an integer multiple of the fundamental wave f1. For example, the third order vibration and fifth order vibration are expressed as f3=3f1 and f5=5f1, respectively.
That is, a piezoelectric body oscillates with the fundamental vibration wherein the thickness of the piezoelectric body is half wavelength and the vibrations corresponding to odd multiples, such as three multiple, five multiple, seven multiple, or the like, of the fundamental vibration. Using the higher harmonic vibration, a resonator having more 16 MHz is realized. However, since the resonator using the higher harmonic vibration has small amplitude, driving voltage for oscillating becomes higher and frequency jump due to the oscillation in fundamental vibration occurs.
Also, even if the higher harmonic vibrations of over third order are used, since the thickness of the piezoelectric body must be thin in order to obtain the oscillating frequency of over 50 MHz, processibility or workability deteriorates. Thus, it is difficult to fabricate piezoelectric elements.